Method for reducing channel change startup delays for multicast digital video streams

RELATED APPLICATIONS

The U.S. patent application Ser. No. 10/969,113, filed Oct. 20, 2004, entitled “SYSTEM AND METHOD FOR FAST START-UP OF LIVE MULTICAST STREAMS TRANSMITTED OVER A PACKET NETWORK,” by Ramanathan T. Jagadeesan, Kristen Marie Robins, Bich Tu Nguyen, and Fang Wu, assigned to the same assignee of the present Patent Application, is hereby incorporated by reference.

TECHNICAL FIELD

Embodiments of the present invention pertain to digital video transport streams, and in particular to a method of reducing the startup delay for multicast digital video transport streams.

BACKGROUND

As video transmission systems have matured, digital video is more readily available via a variety of different communications systems and networks. Specifically, digital video, such as television programs, can be transmitted as multicast digital bit streams of video signals to users over networks. Changing between channels of the multicast digital bit streams can often result in significant delays and interruption in user presentation experience.

Multicast digital bit streams typically include digital video frames. A predetermined number of frames is conventionally referred to as a Group of Pictures (GOP). The GOP lengths are typically 15 or 30 frames. With more advanced video formats, such as Advanced Video Coding (AVC) and/or Windows Media 9 (WM9), the GOP length can be substantially longer in order to reduce the bit rate.

In order to reduce costs and simplify the amount of effort associated with video transmission, different video compression/de-compression techniques have been developed and established. Some of the better known and more widely adopted video compression/de-compression standards include Motion Picture Experts Group 2 (MPEG-2) data streams and Motion Picture Experts Group 4 (MPEG-4) data streams. Hence, television programs are often transmitted over the network as Motion Picture Experts Group 2 (MPEG-2) data streams or Motion Picture Experts Group 4 (MPEG-4) data streams. Conventionally, for purposes of video compression/decompression, a video stream is processed one frame at a time.

Compressed video transmission streams typically include a variety of different compression frame types. With MPEG-2 and MPEG-4, the bit streams generally include three different types of frames including Intra-frames, Predictive frames, and Bidirectional interpolated frames. In a typical decoding process, Intra-frames (I-frames) can be decoded independently without the need of referencing another frame. Thus, GOPs typically start with an I-frame. Predictive frames (P-frames) can be decoded by referencing a previous I-frame or P-frame. Bidirectional interpolated frames (B-frames) can be predicted from a previous and a following P-frame or I-frame. For a given video stream, all three ways of coding are attempted and the best and most efficient combination is utilized. For example, a common MPEG-2 video stream can be 15 frames long and have the sequence IBBPBBPBBPBBPBB.

Typically, a video stream, such as a MPEG-2 data stream, is transmitted from a multicast source to a router and/or switch via a network, e.g., an Internet Protocol (IP) distribution network. And upon receipt of the video stream, the router then transmits the video stream to a user device, such as a set-top box.

However, unfortunately, live multicast video streams transmitted over a network, e.g., an IP distribution network, are frequently affected by start-up delays when a user changes channel. Start-up delays can be caused by video stream characteristics, buffer fill-up time, and/or the time it takes to acquire video stream specific information. Video stream specific information is a periodic update and interleaves with other data. For example, conventional channel change operations wait to acquire video stream specific information and the beginning of the next GOP before presenting content, which often results in relatively long delays.

In particular, a single factor that can cause a considerable set-top box startup delay is the length of the Group of Picture involved. Traditionally the set-top box startup time is directly related to the lengths of a Group of Picture (GOP) from the new channel. Thus, typically the longer the GOP length, the longer the startup delay time is for a set-top box. As a result, in general, the start-up delay associated with GOP length prevents a user from being able to transition from one channel to another without appreciable delay. Consequently, the multimedia experience is affected and made less enjoyable.

BRIEF DESCRIPTION

<figref idrefs="DRAWINGS">FIG. 1</figref> illustrates an Internet Protocol (IP) television channel change system, in accordance with an embodiment of the present invention.

<figref idrefs="DRAWINGS">FIGS. 2A and 2B</figref> illustrate embodiments of the present invention in operation.

<figref idrefs="DRAWINGS">FIG. 3</figref> illustrates block diagrams of an exemplary speed-up GOP insertion, in accordance with an embodiment of the present invention.

<figref idrefs="DRAWINGS">FIG. 4</figref> illustrates a flowchart of a channel change method upon which embodiments in accordance with the present invention can be implemented.

<figref idrefs="DRAWINGS">FIG. 5</figref> illustrate block diagrams of a Fast Start Transmitter, upon which embodiments of the present invention can be implemented.

DETAILED DESCRIPTION

Reference will now be made in detail to embodiments of the present invention, examples of which are illustrated in the accompanying drawings. While the invention will be described in conjunction with these embodiments, it will be understood that they are not intended to limit the invention to these embodiments. On the contrary, the invention is intended to cover alternatives, modifications and equivalents, which can be included within the spirit and scope of the invention as defined by the appended claims. Furthermore, in the following detailed description of the present invention, numerous specific details are set forth in order to provide a thorough understanding of the present invention. However, it will be evident to one of ordinary skill in the art that the present invention can be practiced without these specific details. In other instances, well known methods, procedures, components, and circuits have not been described in detail as not to unnecessarily obscure aspects of the invention.

Some portions of the detailed descriptions that follow are presented in terms of procedures, logic blocks, processing, and other symbolic representations of operations on data bits within a computer memory. These descriptions and representations are the means used by those skilled in the data processing arts to most effectively convey the substance of their work to others skilled in the art. A procedure, logic block, process, etc., is here, and generally, conceived to be a self-consistent sequence of steps or instructions leading to a desired result. The steps are those requiring physical manipulations of physical quantities. Usually, though not necessarily, these quantities take the form of electrical or magnetic signals capable of being stored, transferred, combined, compared, and otherwise manipulated in a computer system. It has proven convenient at times, principally for reasons of common usage, to refer to these signals as bits, bytes, values, elements, symbols, characters, terms, numbers, or the like.

It should be borne in mind, however, that all of these and similar terms are to be associated with the appropriate physical quantities and are merely convenient labels applied to these quantities. Unless specifically stated otherwise as apparent from the following discussions, it is appreciated that throughout the present invention, discussions utilizing terms such as “setting,” “storing,” “scanning,” “receiving,”

“sending,” “disregarding,” “entering,” or the like, refer to the action and processes of a computer system or similar electronic computing device, that manipulates and transforms data represented as physical (electronic) quantities within the computer system's registers and memories into other data similarly represented as physical quantities within the computer system memories or registers or other such information storage, transmission or display devices.

Embodiments of the present invention pertain to methods and systems for reducing delays in video signal changes. Channel change startup delays associated with multicast digital video streams are reduced by utilizing a speed-up Group of Pictures. In one exemplary embodiment, a speed-up Group of Pictures of minimum length is injected and placed in front of an intra-frame of a normal Group of Pictures to facilitate rapid synchronization of the normal Group of Pictures. The present exemplary embodiment includes receiving a channel change request, such as an Internet Group Management Protocol (IGMP) join request, from a user and determining a packet transmission order for packets of a multicast digital video transport stream. The packet transmission order includes a normal Group of Pictures having a first intra-frame. It is appreciated that the Group of Pictures may be preceded by various information such as Program Clock References, Program Specific Information, Program Association Table (PAT), Program Map Table (PMT), Sequence Header, and Group of Picture Header.

In one embodiment, the method includes inserting a speed-up Group of Pictures (e.g., a single picture Group of Pictures) and video stream specific information, in front of the first intra-frame of the normal Group of Pictures to facilitate rapid rendering of the normal Group of Pictures. The video stream specific information may, in one example, include a recalculated PCR. In the present embodiment, the insertion of the speed-up Group of Pictures is implemented by a Fast Start Transmitter (FST). However, the present invention is not necessarily limited to a FST, and Speed-up GOP insertion can be implemented by other devices (e.g., a router, a switch, and/or a network device).

In addition, the exemplary implementation includes processing the packets in the determined packet transmission order. In the present embodiment, the packets are transmitted in the following order: Recalculated Program Clock Reference (PCR), Program Association Table (PAT), Program Map Table (PMT), Sequence Header, Group of Pictures Header, Intra-frame of the speed-up Group of Pictures, Intra-frame of the normal Group of Pictures. Thus, by inserting a speed-up Group of Pictures and video stream specific information, the present invention allows start-up delay to be significantly reduced.

<figref idrefs="DRAWINGS">FIG. 1</figref> illustrates an Internet Protocol (IP) television channel change system <b>100</b>, in accordance with an embodiment of the present invention. Internet Protocol (IP) television Channel change system <b>100</b> includes a multicast source <b>104</b>, a network <b>106</b>, a router <b>108</b>, a Fast Start Transmitter (FST) <b>112</b>, and an set-top box <b>110</b> (e.g., Internet Protocol set-top box). Within channel change system <b>100</b>, multicast source <b>104</b>, network <b>106</b>, router <b>108</b>, and set-top box <b>110</b> are communicatively coupled.

A set-top box (STB) is an electronic device that is connected to a communication channel, such as a phone, Integrated Services Digital Network (ISDN), Digital Subscriber Line (DSL), or cable television line, and produces output on a display, e.g., conventional television screen. Set-top boxes are commonly used to receive and decode digital television broadcasts. Set-top boxes fall into several categories, from the relatively simple implementations that receive and unscramble incoming television signals to more complex implementations that can also function as multimedia desktop computers. The multimedia desktop computers can run a variety of advanced services such as videoconferencing, home networking, IP telephony, video-on-demand (VoD) and high-speed Internet TV services. In one embodiment, a set-top box (STB) is an Internet Protocol (IP) set-top box (STB) which receives data in the form of Internet Protocol (IP) packets.

Although channel change system <b>100</b> is shown and described as having certain numbers and types of elements, the present invention is not necessarily limited to the exemplary implementation. That is, channel change system <b>100</b> can include elements other than those shown, and can include more than one of the elements that are shown. For example, channel change system <b>100</b> can include a greater or fewer number of sources than the one source (multicast source <b>104</b>) shown.

Further, the network <b>106</b> of channel change system <b>100</b> can be implemented in a wide variety of ways. For example, network <b>106</b> can include various local area networks (LAN), metropolitan area networks (MAN), wide area networks (WAN), and/or the Internet. It is appreciated that components of channel change system <b>100</b> can be communicatively coupled via wired and/or wireless communication technologies. It is also appreciated the present invention is not necessarily limited to the configuration of channel change system <b>100</b>.

In the present embodiment, video programs are forwarded by multicast source <b>104</b>, which encodes the media content and sends packets of a multimedia stream through network <b>106</b>. The multimedia stream can be compatible with a variety of compression protocols, for example, Motion Picture Experts Group 2 (MPEG-2) format, Motion Picture Experts Group 4 (MPEG-4) format, and/or any other current or future video compression standards. Upon receipt of the multimedia stream packets, multicast source <b>104</b> sends multimedia stream packets to router <b>108</b> in network <b>106</b>, (e.g., an IP network). The MPEG-2 or MPEG-4 packets can be encapsulated within Real-Time Transport Protocol (RTP) and/or User Datagram Protocol (UDP) packets.

Also, although in the present embodiment only router <b>108</b> is shown, the present invention can be implemented to include other multicast-capable routers, switches, and IP network devices as part of the multicast distribution tree. In one embodiment, router <b>108</b> includes a Fast Start Transmitter <b>112</b> (FST).

Upon receipt of the multimedia stream packets, in one example, router <b>108</b> forwards multimedia stream packets to set-top box <b>110</b> that joins the multicast stream. Additionally, in other embodiments, it is appreciated that router <b>108</b> can send multimedia stream packets to other end stations that are part of the multicast group. An end station can join a multicast group by sending an Internet Group Management Protocol (IGMP) join request to router <b>108</b>.

In one implementation, a user can send a channel change request from a remote control to set-top box <b>110</b>. Set-top box <b>110</b> receives the channel change request and forwards to router <b>108</b>. Upon receipt of the channel change request, router <b>108</b> determines the packet processing sequence for the multimedia stream packets of the new channel. The packet processing sequence includes a normal Group of Pictures, and the normal Group of Pictures includes a first intra-frame.

In one embodiment, Fast Start Transmitter <b>112</b> (FST) of router <b>108</b> generates an accelerating or speed-up Group of Pictures, e.g., a Group of Pictures comprising a single Intra-frame. In one embodiment, the FST inserts the accelerating Group of Pictures in front of the first normal GOP intra-frame to facilitate rapid picture rendering of the normal Group of Pictures.

In one embodiment, the Fast Start Transmitter (FST) includes one or more Startup Transmit Buffers (STX_BUFs) that function to buffer past packets from one or more multicast streams. Startup transmit buffering of multicast media streams can be implemented as a configured option on network device such as a router or switch. The startup transmit buffers can either be fixed in size, or be of a dynamic size, based on application specific-requirements, and can contain a continuous stream or “sliding window” of prior received multicast information. Alternatively, the startup transmit buffers can contain information deemed important in a piecemeal fashion from received multicast packets according to application specific requirements.

In addition, the U.S. patent application Ser. No. 10/969,113, filed Oct. 20, 2004, entitled “SYSTEM AND METHOD FOR FAST START-UP OF LIVE MULTICAST STREAMS TRANSMITTED OVER A PACKET NETWORK,” by Ramanathan T. Jagadeesan, Kristen Marie Robins, Bich Tu Nguyen, and Fang Wu, assigned to the same assignee of the present Patent Application, is hereby incorporated by reference, which further describes functions that can be performed by a Fast Start Transmitter (FST) and a Startup Transmit Buffers (STX_BUFs)

Specifically, when a user joins the multicast media stream, the FST operates to insert a speed-up GOP and transmit (e.g., in a burst mode) the transport packets stored in the STX_BUF to the set-top box (STB). In one embodiment, the FST inserts a speed-up GOP in a head of a normal GOP in the STX_BUF. The set-top box uses the speed-up GOP and normal GOP burst packets to generate a sufficient buffer of information that can be used to quickly start the rendering of media content.

Thus, the FST, by generating and inserting an accelerating or rapid synchronization GOP in front of the first Intra-frame, facilitates rapid picture rendering of the normal GOP and start-up delay associated with Group of Pictures length is significantly reduced.

<figref idrefs="DRAWINGS">FIGS. 2A and 2B</figref>, illustrate embodiments of the present invention in operation. <figref idrefs="DRAWINGS">FIG. 2A</figref> illustrates a normal GOP with multimedia stream packets. However, startup delay associated with GOP length is not yet addressed.

<figref idrefs="DRAWINGS">FIG. 2B</figref> illustrates the insertion of a speed-up GOP <b>204</b> and video stream specific information <b>206</b> in front of the normal GOP <b>202</b>. In one embodiment, the speed-up GOP <b>204</b> is inserted in the head of a normal GOP in the STX_BUF. Insertion of the speed-up GOP <b>204</b> allows rapid synchronization of normal GOP <b>202</b> and reduces start-up delay associated with GOP length. For example, delays otherwise associated with extraneous intermediate frames (e.g., P-frame and/or B-frames) can be reduced. By inserting the speed-up GOP <b>204</b> including only one Intra-frame and video stream specific information <b>206</b> (e.g., a recalculated PCR, PAT, and PMT), start-up latency associated with GOP length can be significantly reduced.

<figref idrefs="DRAWINGS">FIG. 3</figref> illustrates block diagrams of an exemplary speed-up GOP insertion, in accordance with an embodiment of the present invention. A first multicast digital video transport stream <b>320</b> corresponds to a first content channel <b>1</b>, a second multicast digital video stream <b>322</b> corresponds to a second content channel <b>2</b>, and a third multicast digital video stream <b>324</b> corresponds to a Fast Start Transmitter (FST) frame sequence. Multicast digital video transport stream <b>320</b> includes Channel <b>1</b> GOP <b>302</b>, Channel <b>1</b> GOP <b>304</b>, Channel <b>1</b> GOP <b>306</b>, and Channel <b>1</b> GOP <b>308</b>. Multicast digital video transport stream <b>322</b> includes Channel <b>2</b> GOP <b>310</b>, Channel <b>2</b> GOP <b>312</b>, Channel <b>2</b> GOP <b>314</b>, and Channel <b>2</b> GOP <b>316</b>. Multicast digital video transport stream <b>324</b> includes Channel <b>2</b> video stream specific information <b>309</b>, Speed-up GOP <b>310</b>, Channel <b>2</b> GOP <b>312</b>, Channel <b>2</b> GOP <b>314</b>.

A user, such as an Internet Protocol Television (IPTV) viewer watching IPTV from channel <b>1</b>, can initiate a channel change at time T<b>1</b>. Time T<b>1</b> corresponds to the point between the second B-frame and first P-frame of channel <b>1</b> GOP <b>304</b>. Similarly, Time T<b>1</b> corresponds to the point between the first P-frame and third B-frame of channel <b>2</b> GOP <b>312</b>.

Traditionally, a channel change request at Time T<b>1</b> from multicast digital video transport stream <b>320</b> (channel <b>1</b>) to multicast digital video stream <b>322</b> (channel <b>2</b>) includes sending the remaining number of P-frames from channel <b>2</b> GOP <b>312</b> before sending the I-frame from channel <b>2</b> GOP <b>314</b>. Because a GOP usually includes one I-frame and 14 Bidirectional interpolated and Predictive frames, the remaining number of B-frames and P-frames from channel <b>2</b> GOP <b>312</b>, in this example, is 11 frames, which can cause a significant channel change time delay.

In contrast to traditional approaches, the present invention facilitates rapid synchronization of normal GOP channel changes by inserting a Speed-up GOP in front of the normal GOP. Specifically, in the present embodiment, in response to a channel change request, a network device (e.g., a Fast Start Transmitter) inserts video stream specific information <b>309</b> and Speed-up GOP <b>310</b> in front of Channel <b>2</b> GOP <b>312</b> to facilitate rapid synchronization of Channel <b>2</b><b>324</b> at an IP Set-top Box.

The present invention facilitates a more rapid synchronization with channel <b>2</b><b>324</b> because instead of synchronizing with channel <b>2</b> GOP <b>314</b> by processing <b>11</b> remaining B-frames and P-frames of GOP <b>312</b>, the present invention allows synchronization with channel <b>2</b> GOP <b>312</b> by processing the video stream specific information <b>309</b> and speed-up GOP <b>310</b> (e.g., a single I-frame) alone. Consequently, the synchronization time can be reduced appreciably.

It is appreciated that speed-up GOP insertion can be implemented in a variety of configurations. For example, buffer capabilities of a STX_BUF buffer can be utilized to insert speed-up GOP in front of GOP <b>312</b>.

<figref idrefs="DRAWINGS">FIG. 4</figref> illustrates a flowchart <b>400</b> of a channel change method upon which embodiments in accordance with the present invention can be implemented. Although specific steps are disclosed in flowchart <b>400</b>, such steps are exemplary. That is, embodiments of the present invention are well suited to performing various other or additional steps or variations of the steps recited in flowchart <b>400</b>. It is appreciated that the steps in flowchart <b>400</b> can be performed in an order different than presented.

At block <b>404</b>, packets of a multicast digital video transport stream having a plurality of normal Group of Pictures are received. In one example, the packets are received from a multicast source by a FST (fast start transmitter). It is appreciated that the multimedia digital video transport stream can be in Motion Picture Experts Group 2 (MPEG-2) format or in Motion Picture Experts Group 4 (MPEG-4) format.

At block <b>406</b>, a channel change request is received. In one embodiment, the channel change request is an Internet Group Management Protocol (IGMP) join request. In other embodiments, the channel change request can be other applicable join requests.

At block <b>408</b>, a speed-up Group of Pictures (GOP) is inserted in the stream (e.g., in front of the first I-frame) in response to the channel change request. Also, video stream specific information, such as PMT, PAT, and a modified PCR, may be inserted in the stream. The speed-up GOP can be inserted in front of the first Intra-frame by different devices. In one instance, the speed-up GOP can be created and inserted by a Fast Start Transmitter (FST). In another instance, the speed-up GOP can be created and inserted by other network devices. A speed-up GOP can be implemented in different ways. In one example, a speed-up GOP includes a single blank Intra-frame. In another example, a speed-up GOP includes a single Intra-frame that is identical to a subsequent Intra-frame.

At block <b>410</b>, the packets are processed. In one example, processing includes reassembling and transmitting packets. In one embodiment, after the speed-up GOP is inserted, the transmission order is as follows: Recalculated Program Clock Reference PCR, Program Association Table (PAT), Program Map Table (PMT), Sequence Header, Group of Pictures Header, Intra-frame of the speed-up Group of Pictures, Intra-frame of the normal Group of Pictures. Thus, injecting a speed-up GOP, the IP set-top box can synchronize the new video stream faster and start-up delay associated with GOP length is appreciably reduced.

<figref idrefs="DRAWINGS">FIG. 5</figref> is a block diagram that illustrates a Fast Start Transmitter (FST) <b>500</b> upon which embodiments of the present invention can be implemented. System <b>500</b> includes a control unit <b>502</b>, an input port <b>504</b> for receiving data, an output port <b>506</b> for sending data, a memory <b>508</b>, and a startup transmit buffer (STX_BUF) <b>520</b>. Also, system <b>400</b> includes PCRs <b>510</b>, PAT <b>512</b>, PMT <b>522</b>, MPEG Headers <b>514</b>, Speed-up GOP <b>516</b>, and Raw Data <b>518</b>. Although Fast Start Transmitter (FST) <b>500</b> is shown and described as having certain numbers and types of elements, the present invention is not necessarily limited the exemplary implementation. That is, FST <b>500</b> can include elements other than those shown, and can include more than one of the elements that are shown.

In FST <b>500</b>, in one embodiment, control unit <b>502</b> controls input port <b>504</b> for receiving data into memory <b>508</b>. Also, control unit <b>502</b>, in one example, controls storage of incoming data into Raw Data buffer <b>518</b>. Further, control unit <b>502</b> can create PCRs <b>510</b>, PAT <b>512</b>, PMT <b>522</b>, Headers <b>514</b>, and Speed-up GOP <b>516</b> for outgoing data. In one implementation, control unit <b>502</b> can store data, such as PCR <b>510</b>, PAT <b>512</b>, and PMT <b>522</b> in Output Buffer <b>520</b>. Moreover, in one embodiment, control unit <b>502</b> controls Output Port <b>506</b> for data transmission. The control unit <b>502</b> can be implemented in a variety of ways. For example, control unit <b>502</b> may be a CPU, an ASIC, a FPGA and/or a discrete hardware/firmware.

Input Port <b>504</b> receives packets of a multicast digital video transport stream including a plurality of normal Group of Pictures. The multimedia digital video transport stream can be in Motion Picture Exports Group 2 (MPEG-2) format or in Motion Picture Experts Group 4 (MPEG-4) format. In one embodiment, the multicast digital video transport streams are inputted into Input Port <b>504</b> and then into Memory <b>508</b> for further processing. The Raw Data buffer can be fixed in size or of a dynamic size, based on application specific requirements, and can include a continuous stream data. In one example, a buffer for PCRs <b>510</b>, PAT <b>512</b>, PMT <b>522</b>, Headers <b>514</b>, and/or Speed-up GOP <b>516</b> can include relevant information received directly from multicast packets.

In one exemplary implementation of a Speed-up GOP, the multicast streaming data packets are received and stored in Raw Data buffer <b>518</b>. In one example, the data is staged in the buffer but not immediately transmitted. After receiving a channel change request, for example, an ICMP Report (Join) message, a process in Control Unit <b>502</b> is activated. This process, in one embodiment, creates a new GOP of minimum size and inserts it in Speed-up GOP buffer <b>516</b>. Further, in one example, necessary changes are implemented on other piecemeal information (e.g., PCRS, PAT and PMT, stream Headers of various kinds) to meet requirements for an individual stream. Moreover, in one example, a suitable transmission order is determined for transferring the stream over a network, such as a LAN. In one example, the transmission order is PCRs, PAT, PMT, Headers, Speed-up GOP, normal GOP with leading I-frame, and other streaming data.

In one embodiment, FST <b>500</b> can be embedded within a network device. For example, FST can transmit data over Local Area Network (LAN) if embedded in a network router or switch. In another example, FST <b>500</b> can be embedded in a Digital Subscriber Line Access Multiplexer (DSLAM) and/or in a Cable Modem Termination System (CMTS).

To summarize, embodiments, by inserting a Speed-up GOP and video stream specific information in front of a normal stream, trigger client devices to start decoding much sooner, and significantly reduce channel change delay for live video streaming. Thus, embodiments improve a user's viewing experience to be more pleasant and positive.

In the foregoing specification, embodiments of the invention have been described with reference to numerous specific details that can vary from implementation to implementation. Thus, the sole and exclusive indicator of what is, and is intended by the applicants to be the invention is the set of claims that issue from this application, in the specific form in which such claims issue, including any subsequent correction. Hence, no limitation, element, property, feature, advantage or attribute that is not expressly recited in a claim should limit the scope of such claim in any way. The specification and drawings are, accordingly, to be regarded in an illustrative rather than a restrictive sense.